The broad objective of this proposal for continuation of R01CA79804 is to elucidate the role of MDM2 on cell proliferation and tumorigenesis. MDM2 has p53-dependent and p53-independent oncogenic functions. While MDM2 p53-dependent function has been intensively studied, the p53-independent functions of MDM2 are not well understood. This application focuses on MDM2-mediated regulation on retinoblastoma protein (Rb). Work from us and others showed that MDM2 interacts with Rb and promotes S-phase entry independent of p53. We recently showed that MDM2 binds to the Rb C-pocket and disrupts Rb-E2F interaction. Furthermore, we demonstrated that activation of MDM2 promotes Rb protein degradation in a proteasome-dependent pathway whereas knockdown of MDM2 results in accumulation of Rb and cell cycle arrest. In this continuation application, we hypothesize that MDM2-mediated Rb destabilization critically contributes to pathological progress of tumorigenesis. Specific Aim 1 is to elucidate the molecular mechanism by which MDM2 regulates Rb. Specific Aim 2 is to investigate the roles of MDMx and tumor suppressor protein ARF in modulation of MDM2-Rb and Specific aim 3 is to examine the role of MDM2-Rb in tumorigenesis in vivo. Information obtained from this study will provide considerable insight of the biological and pathological role of MDM-Rb in cell proliferation and human tumorigenesis. Inactivation of retinoblastoma protein (Rb) is critical in human cancer development. Rb can be inactivated by gene mutations and by abnormal hyperphosphorylation. How Rb is degraded is poorly understood. Work from us and others demonstrates that the cellular oncoprotein MDM2 also targets and promotes Rb protein degradation. This study is aimed at elucidating the role of he MDM2-Rb pathway in human tumorigenesis. Information obtained from this study would further our understanding of human carcinogenesis and help identify therapeutic drug targets for cancer prevention and treatment.